This invention relates to a surface acoustic wave device wherein mechanical reflection of a surface acoustic wave is reduced.
A surface acoustic wave device, which mainly consists of a piezoelectric substrate made of a piezoelectric single-crystal material such as crystal, a piezoelectric ceramic material or a combination of a non-piezoelectric plate and a piezoelectric film material deposited thereon, is arranged to convert an electric signal to a surface acoustic wave by a transducer provided on the piezoelectric substrate and to propagate the surface acoustic wave along the substrate surface. It is now employed as filters and other various electronic parts.
FIG. 1 shows a filter as an example of the electronic parts in which reference numeral 1 designates a piezoelectric substrate, 2 is an input tranducer comprising a pair of interdigitating comb-shaped electrodes 2A and 2B, and 3 is an output transducer comprising a pair of interdigitating comb-shaped electrodes 3A and 3B. An electric signal applied to an input terminal IN is converted to a surface acoustic wave by means of the input transducer 2 and travels along the surface of the piezoelectric substrate 1 as shown by the arrow in the Figure. When the surface acoustic wave reaches the output transducer 3, it is reconverted to an electric signal by the transducer 3 and is taken out from an output terminal OUT. The transducers 2 and 3 are so-called normalized type of electrodes with each electrode strip width W and each distance L between adjacent electrode strips of the electrodes 2A, 2B, 3A and 3B are .lambda./4 when wavelength of the center frequency f.sub.o of a surface acoustic wave is .lambda..
Since the filter including the transducers comprising the comb-shaped electrodes with the above-mentioned measurement cannot be free from multiple reflection i.e. TTE (triple transit echo) of a surface acoustic wave between the input and output transducers during their operations, the wave after passing through the filter is out of order in its phase (ripple is caused). This is undesirable for reception of FM signals, for example.
There are two factors of generating said TTE. One of the factors is mechanical reflection of a surface acoustic wave due to difference between an acoustic impedance in a zone where the comb-shaped electrodes are located and an acoustic impedance in a zone where the electrodes do not exist. The other factor is electric reflection due to bidirectional property of the transducers (a nature that the transducers are capable of transmitting or receiving surface acoustic waves symmetrically right and left). As to the electrical reflection, its influences can be reduced by causing on purpose mis-matching at the input and output of the transducers, or by changing the transducers to be unidirectional by a polyphase power supply method. As to the mechanical reflection, however, irrespective of the following various trials, satisfactory effects could not be obtained.
One of the trials is to design the comb-shaped electrodes 2A, 2B, 3A and 3B of the input and output transducers 2 and 3 by dividing each strip into two parts so that each divided electrode strip width W and distance L between the strips are .lambda./8, respectively, as shown in FIG. 2.
According to the structure, since the phases of the reflected waves at the respective electrode ends differ by 180.degree. from each other, that is, they are opposite phases, the reflected waves mutually cancel each other, thereby certainly reducing undesired influences by the mechanical reflection.
However, the electrode strip width W and the distance L between the electrode strips must be .lambda./8, and higher the frequency, smaller the .lambda. becomes. Therefore, extremely high accuracy is required upon making the electrodes by photolithographic method, thereby leading to lower productivity in manufacturing of the device.
Further, since the distance between the electrode strips is extremely small, short circuit between the electrodes easily occurs due to dust or other particles, thereby causing mis-operation of the device.
Another trial is to employ a structure of the piezoelectric substrate 1 comprising an elastic material plate 4 and a piezoelectric film 5 covering the plate 4 as shown in FIGS. 3(a) and 3(b). A sheet-like lower electrode 6 to serve as one of the above-mentioned comb-shaped electrodes is provided between the plate 4 and the piezoelectric film 5 whereas upper electrodes 7A and 7B to serve as the other of the electrodes are provided on the piezoelectric film 5 so that they are opposed to the lower electrode 6 and that the electrode strip width W and the distance L between the respective electrode strips are .lambda./2, respectively. Thereby, a single phase transducer is constructed.
This structure of the transducer is different from a normalized electrode and includes only one comb-shaped electrode 7A or 7B so that a signal source 8 is connected between the upper electrode 7A and the lower electrode 6 to take out an electric signal at a load 9 between the upper electrode 7B and the lower electrode 6. Therefore, bad influences to the electrodes by dust or other particles are prevented and accuracy for making the electrodes may be lower than the former case, thereby improving productivity in manufacturing of the device.
However, since this structure makes it impossible to drive the device by the so-called balanced power supply by supplying the upper electrode 7A with signals having two polar (positive and negative) electric potentials with respect to the potential (earth potential) of the lower electrode 6, electromagnetic coupling (feedthrough) between the input and output transducers, wherein the electric signal is not converted to a surface acoustic wave and is propagated as a direct wave from the input to the output, becomes large. As the result, the direct wave and the necessary surface acoustic wave concurrently exist, thereby worsening filtering effect.
A still further improvement is to design a so-called balanced-type single phase transducer wherein further upper electrodes 8A and 8B are opposed to the upper electrodes 7A and 7B at .lambda./2 phase difference and via the space a as shown in FIG. 4, in order to enable the balanced power supply in the structure of FIG. 3.
This structure certainly enables reduction of feedthrough by supplying signals at 180.degree. phase difference between the upper electrodes 7A and 7B from a power source 8.
However, since this structure requires the space a between the both electrodes for prevention of short circuit, surface acoustic waves S.sub.1 and S.sub.2 which must have a same phase as a result of excitation by the electrodes come to disorder in their waveforms due to existence of the space a which does not generate a surface acoustic wave.